1. Field of the Invention
Embodiments of the present invention generally relate to light-emitting diode (LED) semiconductor processing and, more particularly, to a method of producing vertical light-emitting diode (VLED) dies comprising Group III-Group V combinations of elements and a metal substrate.
2. Description of the Related Art
To produce blue, green, and ultraviolet vertical light-emitting diodes (VLEDs), various semiconductor materials are utilized, typically comprising combinations of elements from the Group III and Group V areas of the periodic table. Nitrogen being the most prevalent Group V element for such VLEDs, these combinations generally include GaN, AlN, AlGaN, and InGaN. However, layers of Group III-N compound semiconductor materials cannot simply be formed with high crystalline quality on conventional substrates of silicon, germanium, or GaAs, so epitaxial methods were developed using sapphire or silicon carbide (SiC) as a substrate instead.
Even still, the crystallinity and corresponding light emission efficiency of the semiconductor layers produced by these methods leaves room for improvement. Researchers discovered that forming a GaN or AlGaN buffer layer on the substrate before growing additional compound semiconductor layers improved the crystallinity and the light emission as disclosed in U.S. Pat. No. 5,290,393. To improve the thermal conductivity of VLEDs, however, a metal substrate may be coupled to the light-emitting diode (LED) stack, and the growth-supporting substrate of sapphire or SiC may be subsequently removed.
One technique for removing the growth-supporting substrate 102 involves laser lift-off as disclosed in U.S. published patent application 2006/0154389. This laser lift-off technique is illustrated in FIG. 1 where laser pulses 104 are implemented to sever the boundary between the substrate 102 and the buffer layer 106. With laser lift-off, however, shockwaves 108 from the laser pulses may travel throughout the LED stack 110 and may damage the critical light-emitting active layer 112. Damage to the active layer 112 may significantly reduce the brightness of the VLED as more defects could be introduced into the active layer in the form of non-radiative recombination centers.
Furthermore, intentional removal of the GaN or AlGaN buffer layer after removal of the growth-supporting substrate may cause further damage to the device. FIG. 2 is a microscope image 202 and a roughness analysis 204 of the surface of a VLED after removal of a GaN buffer layer. The atomic force microscope (AFM) image 202 appears noticeably rough, and the maximum depth 206 of 115.77 nm indicates the degree of damage to the crystalline structure of the VLED surface. The damage to the surface of the VLED may also be responsible for reduced brightness since shockwaves produced during removal may reach the active layer.
Accordingly, what is needed is an improved method of producing VLED dies.